diff --git a/src/ca/cac.cpp b/src/ca/cac.cpp
index 82a24345f..9fa6374b1 100644
--- a/src/ca/cac.cpp
+++ b/src/ca/cac.cpp
@@ -1619,26 +1619,24 @@ void cac::waitUntilNoRecvThreadsPending ()
         {
             fd_set mask;
             FD_ZERO ( & mask );
-            int count = 0;
+            SOCKET maxFD = 0;
             epicsGuard < cacMutex > guard ( this->mutex );
             tsDLIter < tcpiiu > iter = this->serverList.firstIter ();
             if ( this->pudpiiu ) {
-                this->pudpiiu->fdMaskSet ( mask );
-                count++;
+                this->pudpiiu->fdMaskSet ( mask, maxFD );
             }
             while ( iter.valid() ) {
-                iter->fdMaskSet ( mask );
-                count++;
+                iter->fdMaskSet ( mask, maxFD );
                 iter++;
             }
 
             struct timeval delay = { 0, 0 };
-            int status = select ( count, & mask, 0, 0, & delay );
+            int status = select ( maxFD+1, & mask, 0, 0, & delay );
             if ( status <= 0 ) {
                 return;
             }
             this->nRecvThreadsPending = 
-                static_cast < unsigned > ( count );
+                static_cast < unsigned > ( status );
         }
 
         this->recvThreadActivityComplete.wait ( 0.1 );
diff --git a/src/ca/udpiiu.h b/src/ca/udpiiu.h
index c063c75d6..e40b01b63 100644
--- a/src/ca/udpiiu.h
+++ b/src/ca/udpiiu.h
@@ -36,6 +36,7 @@
 #include "epicsThread.h"
 #include "epicsMemory.h"
 #include "epicsTime.h"
+#include "tsMinMax.h"
 
 #ifdef udpiiuh_accessh_epicsExportSharedSymbols
 #   define epicsExportSharedSymbols
@@ -98,7 +99,7 @@ public:
     bool pushDatagramMsg ( const caHdr &hdr, const void *pExt, ca_uint16_t extsize);
     void shutdown ();
     double roundTripDelayEstimate () const;
-    void fdMaskSet ( fd_set & mask ) const;
+    void fdMaskSet ( fd_set & mask, SOCKET & maxFD ) const;
 
     // exceptions
     class noSocket {};
@@ -222,8 +223,9 @@ inline double udpiiu::roundTripDelayEstimate () const
     return this->rtteMean;
 }
 
-inline void udpiiu::fdMaskSet ( fd_set & mask ) const
+inline void udpiiu::fdMaskSet ( fd_set & mask, SOCKET & maxFD ) const
 {
+    maxFD = tsMax ( this->sock, maxFD );
     FD_SET ( this->sock, & mask );
 }
 
diff --git a/src/ca/virtualCircuit.h b/src/ca/virtualCircuit.h
index ba3648b4f..81a4752a3 100644
--- a/src/ca/virtualCircuit.h
+++ b/src/ca/virtualCircuit.h
@@ -28,6 +28,7 @@
 
 #include "epicsMemory.h"
 #include "tsDLList.h"
+#include "tsMinMax.h"
 
 #include "comBuf.h"
 #include "caServerID.h"
@@ -133,7 +134,7 @@ public:
 
     bool bytesArePendingInOS () const;
 
-    void fdMaskSet ( fd_set & ) const;
+    void fdMaskSet ( fd_set &, SOCKET & maxFd ) const;
 
 private:
     hostNameCache hostNameCacheInstance;
@@ -249,8 +250,9 @@ inline unsigned tcpiiu::channelCount ()
     return this->channelList.count ();
 }
 
-inline void tcpiiu::fdMaskSet ( fd_set & mask ) const
+inline void tcpiiu::fdMaskSet ( fd_set & mask, SOCKET & maxFD ) const
 {
+    maxFD = tsMax ( this->sock, maxFD );
     FD_SET ( this->sock, & mask );
 }